Method for producing fullerene-containing carbon and device for carrying out said method

ABSTRACT

The inventive method for producing fullerene-containing carbon and the device for carrying out said method relate to the chemical industry and are used for producing fullerenes. Said method for producing fullerene-containing carbon consists in vaporising graphite in an electric arc between coaxial graphite electrodes. The graphite electrode is continuously moved inside the electric arc zone through the glow discharge zone. The products formed inside the electric arc are removed therefrom with the aid of an annular inert gas flow which is directed along the axes of the electrodes through an area arranged at a defined distance from a discharge axis. The inventive device for producing fullerene-containing carbon comprises a plasma reactor provided with a system for circulating inert gas and a system for recuperating fullerene carbon. Said reactor is provided with a chamber for degassing the graphite electrode which is continuously moved by the glow discharge towards the arc. The system for circulating inert gas is provided with an annular slot nozzle coaxially arranged in relation to the moving graphite electrode.

FIELD OF THE INVENTION

[0001] The present invention relates to manufacturing afullerene-containing soot, the product containing a new allotrope ofcarbon in the form of molecules consisting of 60, 70, 84, or more carbonatoms that is finding application in chemistry, physics, industry, powerengineering, electronics, biology, medicine and other fields.

DESCRIPTION OF THE PRIOR ART

[0002] Since 1985, when a team of scientists from Rice University (see:Kroto H. W., Heath J. R., O'Brien S. C., Curl R. F. and Smalley R.E.—“C₆₀: Buckminster-fullerene”—Nature.—v. 318, No. 6042, p.p. 162-163,Nov. 14, 1985) deciphered the structure of the fullerene molecule, agreat variety of methods for manufacturing fullerenes and designs ofapparatus for realization of these methods have been developed. However,no methods and apparatus have yet been suggested for manufacturingfullerene-containing products, in particular, the fullerene-containingsoot, with high yield and at low cost.

[0003] One method is known of making the fullerene-containing soot, inwhich a graphite powder is introduced into a microwave gas dischargezone, where the graphite powder is vaporized and fullerenes are formedfrom the carbon vapor (see: Patent of Japan No. 05-238717, IPC C 01 B31/02, filed 17.09.93).

[0004] Disadvantages of this method for fullerene-containing sootmanufacturing are the low process output, low content of fullerenes inthe soot, and high electricity consumption per gram of soot as well asthe complexity of technical problems involved in microwave generation.

[0005] An apparatus is known for making the fullerene-containing sootcomprising partially open enclosure capable of withstanding hightemperatures, where an arc discharge is made to burn between graphiteand tungsten electrodes. Condensation of carbon proceeds in a stream ofa noble gas such as He, Ar, or Kr flowing through a horizontal tubeplaced in a magnetic field (see: Patent of Germany No.24205296, IPC C 01B 31/02, filed Aug. 26, 1993).

[0006] The authors of that invention stated that with their apparatusthe yield of fullerenes could be increased to 55%; however, this highyield value had not been confirmed in experiments. A disadvantage ofthis apparatus for making the fullerene-containing soot is low output.

[0007] Another method is known of making the fullerene-containing sootin which carbon is evaporated in a helium atmosphere by applying a d.c.voltage between two opposite graphite electrodes, one being anode andthe other cathode, a helium flow is circulated, and thefullerene-containing soot formed is freed of the contained gas andcollected (see: Patent of Japan No. 05-09013, IPC C 01 B 31/02, filedJan. 19, 1993).

[0008] A disadvantage of the known method of making fullerene-containingsoot is its insignificant yield explained by the fact that about half ofthe carbon vaporized from the anode is depositing not in the zone ofcondensation of the fullerene-containing soot but, instead, on thecathode, in the form of amorphous carbon and various nanostructures(nanotubes, nanoonions, etc.), lowering the yield of soot andcomplicating the maintenance of constant interelectrode distance whenthe method is used in modular installations with a common electrodedisplacement system.

[0009] An apparatus is known for making fullerene-containing sootcomprising a graphite-rod anode of a diameter of 1 mm or more and anopposite electrode-cathode, the diameter of which is twice that of theanode, both placed in a cylindrical enclosure. The is enclosure has twoapertures in it: the lower aperture for introducing helium and the upperone for removal of the gas contained in the carbon powder. The housingis also supplied with a suction-type collector for collecting the powdergenerated (see: Patent of Japan No. 05-09013, IPC C 01B 31/02, filedJan. 19, 1993).

[0010] A disadvantage of the known apparatus is the impossibility ofmaintaining a symmetrical type of gas convection inside, which lowersthe percentage of fullerenes in the soot. This apparatus has inherentlylow soot output because of small diameter of the graphite electrodevaporized.

[0011] A combination of substantial features that is closest to themethod of the present invention for making the fullerene-containing sootis found in a method of making fullerene-containing soot by which acarbon vapor generation zone filled with an inert gas containing firstand second graphite electrodes is provided and a sufficient voltage isapplied between the electrodes to maintain an electric arc; the carbonvapor formed in the arc is carried with a stream of inert gas blownacross the electric arc to a condensation zone where thefullerene-containing soot is collected (see: U.S. Pat. No. 5,227,038,IPC C 01 B 31/00, filed Jul. 13, 1993).

[0012] Disadvantages of the known method of making thefullerene-containing soot are the low output arising from the need toperiodically interrupt the process in order to change the graphite rodsand degass said rods in the reaction zone and distortion of the thermalconditions in the arc by the inert gas circulated across the arc causinga reduction in the fullerene content of the soot.

[0013] The closest to the apparatus of the present invention is anapparatus for making the fullerene-containing soot comprising a plasmareactor in the form of a sealed cylindrical evaporation chamber made ofstainless steel where two graphite electrodes are arranged horizontallyopposite one another. The first electrode via a water-cooled currentfeedthrough is connected to a voltage source. The second electrode isgrounded via another water-cooled current feedthrough. The graphiteelectrodes are fixed in gears for displacement along their common axisto maintain the interelectrode gap width required for the arc. Theapparatus also has an inert gas circulation system and means forentrapment of the soot formed in the burning arc. The housing of theevaporation chamber can be water-cooled (see: U.S. Pat. No. 5,227,038,IPC C 01 B 31/00, filed Jul. 13, 1993).

[0014] Disadvantages of the known prototype apparatus are low outputcaused by the necessity to periodically interrupt the process forreplacement and degassing of the electrodes and removal of the sootdeposited on the walls; lack of symmetry of the inert gas stream throughthe burning arc causing a reduction of the content of fullerenes in thesoot, low yield of the soot compared with the quantity of graphite burntin the arc because of the loss of about half the carbon vaporized fromthe anode due to its deposition on the cathode in the form of amorphouscarbon and various nanostructures (nanotubes, nanoonions and others),the so-called cathode deposit.

SUMMARY OF THE INVENTION

[0015] The object of the present group of inventions is to develop amethod for making fullerene-containing soot and an apparatus for itsrealization featuring a higher output of the fullerene-containing sootgeneration process in an electric arc, an increase in the percentage offullerenes in the soot, and making the soot manufacturing processcontinuous.

[0016] To achieve the above object, in the method for manufacturing thefullerene-containing soot involving vaporization of graphite in anelectric arc burning between axially aligned graphite electrodes underelectrical voltage placed in an atmosphere of inert gas, transfer of theproducts formed with inert gas and their subsequent condensation in theform of fullerene-containing soot,—in said method the electrodeadvancing into the electric arc zone is first passed through a zone ofelectric glow discharge in an inert gas atmosphere whereas the productsformed in the generation zone are taken away by an annular stream ofinert gas blown along the axis of said electrodes. The annular stream ofinert gas can also be made to twist around the electrode axis for moreefficient removal of the soot from the generation zone. In order toremove the deposit forming on the cathode and thus stabilize theelectric arc position the polarity of electrical voltage applied isalternated. For example, it can be maintained positive for a period of2-15 minutes and then switched to negative for 1-5 minutes.

[0017] To further approach the object of the invention the apparatus formanufacturing the fullerene-containing soot comprising a plasma reactorin the form of a sealed cylindrical chamber, an inert gas recirculationsystem, means for entrapment of the fullerene soot, two graphiterod-shaped electrodes opposite one another and axially aligned with saidchamber, of which the first is fixed in a cooled current feedthrough andthe second is mounted in another cooled feedthrough and can be displacedaxially, said reactor is additionally equipped with an antechamber fordegassing the eroding graphite electrode by means of a glow discharge,the system of inert gas recirculation is supplemented with anannular-slit nozzle axially aligned with the graphite electrodes and thefullerene soot entrapment means is equipped with an electric separatorat the inlet of the inert gas recirculation system. The internal radiusR of the nozzle is preferably not less than 45 mm. The nozzle can bepositioned at a distance of 30-40 mm from the plane passing through themidpoint of the interelectrode gap at a normal to its axis. In order toimpart an axial rotation to the annular stream of inert gas a deflectorcan be mounted in the nozzle. The deflector may consist of strips fixedin the nozzle at an acute angle to its axis or it could be guidingblades or any other known design.

[0018] To the authors' knowledge, there is no information available inpatents or scientific and technical literature of a similar method formanufacturing the fullerene-containing soot in an electric arc or of anapparatus possessing the combination of features of the presentinvention, which, in the opinion of the applicants, is an indicationthat the present group of inventions meets the novelty criterion.

[0019] As demonstrated in research by the authors, treatment of thegraphite electrode by glow discharge in an inert gas atmosphere beforeit enters the electric arc zone and the removal of the products formedusing an annular stream of inert gas flowing parallel to the outside ofthe fullerene generation zone permit realization of a continuous processof fullerene soot generation with high yield of fullerenes because it ispossible in said process to continuously advance the graphite electrodesinto the reactor and to remove the soot particles and fullerenes formedin the gas-plasma stream without interfering with the fullereneformation process.

[0020] This continuous process of manufacturing the fullerene-containingsoot can be performed in the apparatus of the present inventioncomprising a chamber for degassing the movable electrode in a glowdischarge, an inert gas recirculation system with an annular-slit nozzleaxially aligned with the movable graphite electrode, and a means forentrapment of the fullerene soot incorporating an electrical separatormounted at the inlet of the inert gas recirculation system, saidapparatus testifying to a definite innovative level of the presentinventions.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021] The method of the present invention for makingfullerene-containing soot can be illustrated by an example of operationof the apparatus drawn in the figures, where

[0022]FIG. 1 is a schematic of the apparatus for makingfullerene-containing soot.

[0023]FIG. 2 is a general view of the plasma reactor chamber with apartial longitudinal section.

[0024]FIG. 3 is a cross-sectional view of the plasma reactor chamberalong A-A.

[0025]FIG. 4 is a general perspective view of the plasma reactor chamberwith a partial longitudinal section.

[0026] The apparatus for manufacturing fullerene-containing soot (seeFIG. 1) comprises hermetically sealed cylindrical chamber 1 having arecirculation system for inert gas (preferably, helium) consisting ofblower 2 connected via conduit 3 and nipple 4 to an annular nozzle 5;means for fullerene soot entrapment 6 containing an electric separator 7and a bag filter 8. Electric separator 7 is connected via conduit 9 todischarge 10 of chamber 1. The bag filter 8 is connected to the blowerinlet by means of output flange 11. Through known-art Wilson seals (see,for example, B. S. Danilin and V. E. Minaichev, Fundamentals ofdesigning vacuum systems, M., Energia, 1971, p. 221) along the axis ofchamber 1 graphite electrodes 15 and 16 are introduced from oppositesides. Electrode 15 is usually fixed and connected by means of cooledcurrent feedthrough 17 to one of the outputs of the arc power supply(not shown in the drawing). Electrode 16 is mounted in such a mannerthat it can be advanced or withdrawn through the cooled currentfeedthrough 18. Outputs of the arc power supply are electricallyisolated from ground. Chamber 1 is grounded and current feedthroughs 17and 18 are isolated from ground with isolators 19, 20 and 21. Electrode16 is passing antechamber 22, where it is being degassed by the glowdischarge. For this purpose a ring electrode 23 is installed in theantechamber 22 coaxially with the electrode 16. Terminal 24 of the ringelectrode 23 is isolated from ground with isolator 25. Terminal 24 isconnected to the positive output terminal of the glow discharge powersource (not shown in the drawing). The negative output terminal of theglow discharge power source is connected to electrode 16. The output ofthe glow discharge power source is isolated from ground. Antechamber 22is evacuated by fore-vacuum pump 26 and filled with inert gas(preferably, argon) from bottle 27 through inlet needle valve 28.Displacement of the electrode 16 is performed by a displacement system29; its tappet 30 is isolated from electrode 16 with isolator 31. Inorder to prevent the soot formed in the arc discharge from depositing onelectrode 15 the latter is enclosed in a conical screen 32. Chamber 1can be cooled for, which purpose it has a jacket 33 (see FIG. 2) withnipples 34 and 35, jacket 36 with nipples 37 and 38, and jacket 39 withnipples 40 and 41. Nipples 34, 35, 37, 38, 40 and 41 are used forletting in and out the cooling agent (water can be used as a coolingagent). Electric separator 7 has an axial central electrode 42. Housing43 of the electric separator 7 is grounded. In the nozzle 5 (FIG. 3), adeflector 44 can be mounted for twisting the annular stream of inertgas. Deflector 44 may have the form of guiding blades or strips mountedat an acute angle relative to the axis of nozzle 5.

MODIFICATIONS OF THE INVENTION

[0027] The method according to the present invention is performed asfollows. Graphite electrodes 15 and 16 are inserted in feedthroughs 17and 18, respectively. As electrodes, cylindrical graphite bars of adiameter 12 mm and length 400 mm are used. Chamber 1 is grounded andcooled current feedthroughs 17 and 18 are connected to the electric arcpower supply. As a power supply, a rectifier of the type used in weldingmachines having a current reversal capability can be employed. Internalvolumes of chamber 1, blower 2, electric separator 7, bag filter 8,internal pockets of the Wilson seals 12, 13, 14 and the antechamber 22of the apparatus are evacuated to 10⁻² Torr by vacuum pump 26 with aliquid nitrogen trap. Then the internal volume of chamber 1, electricseparator 7, and bag filter 8 of the apparatus are filled with inert gasor a mixture of inert gases to a pressure from 80 Torr to theatmospheric pressure (preferably, 100 Torr). Under continuousfore-vacuum pumping a dynamic inert gas pressure of 0.1 to 10 Torr(preferably, 1 Torr) is established in antechamber 22 using inlet needlevalve 28 connected to the inert gas bottle 27 (preferably, argon).Electrodes 16 and 17 are connected to negative and positive terminals ofthe glow discharge power supply and the glow discharge is initiated at acurrent of 1 to 100 mA (preferably, 10 mA). Pumping out of the internalpockets of Wilson seals 12, 13, and 14 is continued throughout the sootmanufacturing process. Gas blower 2 is turned on. A cooling agent ispumped into feedthroughs 17, 18 and jackets 33, 36 and 39. From ahigh-voltage power supply (not shown in the drawing) a voltage of 8-10kV is applied with the negative terminal connected to the centralelectrode 42 of electric separator 7 and positive to the groundedhousing 43. Electrodes 15 and 16 are connected to negative and positiveterminals, respectively, of the arc discharge power supply. An arcdischarge is initiated between electrodes 15 and 16 and the operatingregime of the arc burning is adjusted (discharge current andinterelectrode separation). Advancement system 29 is turned on and anappropriate progression rate of the tappet 30 is established so as tocompensate the evaporation of electrode 16 and thus maintain constantthe interelectrode gap. Carbon evaporating from the electrode 16 isleaving the arc zone in the radial direction, part of it collecting onelectrode 15 and forming the cathode deposit. Therefore, at regularintervals (preferably, 2-15 minutes), the polarity of electrodes 15 and16 is reversed in order to burn the deposit formed on electrode 15. Atreversed polarity the deposit burns out on electrode 15 but forms onelectrode 16. The time of operation with reversed polarity is chosenlong enough for the deposit on electrode 15 to burn out completely(reversed polarity time of 1-5 minutes is preferable). Then the polarityis switched back.

[0028] After certain portion of the graphite rod 16 has been used up,next graphite rods is connected to its rear end (for this purpose theends of the rods are profiled for male-to-female connection). In thisway continuous operation of the apparatus is achieved. While passingthrough antechamber 22, electrode 16 is degassed by the glow dischargemaintained between electrodes 16 and 23. As the glow discharge voltageis much higher than the arc voltage, reversal of the arc polarity haslittle effect on the glow discharge regime in antechamber 22. Thedegassing products are pumped out of antechamber 22 by fore-vacuum pump26 and the inert gas pressure necessary for the glow discharge ismaintained by means of bottle 27 with the inlet needle valve 28.

[0029] The annular stream of inert gas coming out of nozzle 5 picks upthe carbon transformation products and through nipple 10 and conduit 9carries them to electric separator 7, where they collect on the groundedenclosure 43 in the form of fullerene-containing soot. Due tolow-frequency, low-amplitude vibrations of the walls of electricseparator 7 the soot particles fall off into the bag filter 8. Besides,soot particles that passed the electric separator 7 are entrapped by bagfilter 8 and the inert gas after purification is pumped back intochamber 1 by blower 2 through nozzle 5. When the bag filter 8 is full ofsoot the apparatus is either switched to a different entrapment unit orthe process is stopped for cleaning the bag filter 8. The annular streamof inert gas can be simultaneously rotated around the axis of nozzle 5by deflector 44.

EXAMPLE 1

[0030] Three series of experiments on fullerene-containing sootmanufacture were carried out on a prototype apparatus shown in FIG. 1for three different distances R between the slit of nozzle 5 and the arcaxis (four experiments in each series, data averaged). Axially theannular nozzle 5 was positioned at a distance of 30 mm off the midpointof the interelectrode gap. As electrodes, graphite rods 6 mm in diameterwere used. The arc was maintained with a d.c. forward current of 70 A;as inert gas, helium at a pressure of 100 Torr was used; theinterelectrode distance was maintained constant at 4 mm; and the heliumflow velocity at the exit of the annular nozzle was 5 m/s. The advancingelectrode 16 was degassed by the glow discharge as it passed insideantechamber 22. No voltage was applied to the electric separator 7.Quantity of the soot collected on the walls of electric separator 7 andbag filter 8 as a percentage of the total quantity of produced soot aswell as the fullerene content in the soot were determined. The resultsare shown in Table 1. TABLE 1 R, Quantity of soot collected in theFullerene content Item # mm electric separator and bag filter, % in thesoot, % 1 20 >99.5 <0.5 2 46 99.0 12.1 3 70 95.0 14.2

[0031] From the above data it is seen that even at large R practicallyall soot is carried away by the helium flow and collected in theelectric separator 7 and bag filter 8. However, at small R the fullerenecontent in the soot is low.

EXAMPLE 2

[0032] Experiments were carried out on the apparatus (FIG. 1) with theuse of reversible current supply. In these experiments there no heliumblowing was used and the soot was collected from the walls of chamber 1.Moving electrode 16 and fixed electrode 15 were graphite rods 6 mm indiameter. The helium pressure was 100 Torr; the forward and reverse arcdischarge currents were identical and equal to 80 A. The startinginterelectrode gap was 4 mm. Displacement velocity of the electrode 16in all experiments was practically constant and equal to about 4 mm/min.The fullerene content of about 11-13% in the soot was weakly dependenton arc regime. The results of the experiments are given in table 2.

[0033] Notation in the columns of table 2 is as follows.

[0034] T₁—time period of the forward arc current (moving electrode 16connected to positive terminal of the arc power supply);

[0035] T₂—time period of the reverse arc current;

[0036] κ—number of current reversals in one experiment;

[0037] d_(k)—interelectrode gap at the end of experiment. TABLE 2 T₁,T₂, d_(k), Item # min. min. κ mm 1 1 1 6    7*) 2 15 1 6 1 3 15 5 6 4 410 3.5 20 4

[0038] As seen in Table 2, at a certain ratio of the arc burning timeperiods in forward and reverse polarity periodic voltage polarityreversal prevents the formation of cathode deposit so that theinterelectrode gap and the arc position remain practically constant. Asa result, a continuous fullerene manufacturing process is realized.

INDUSTRIAL APPLICABILITY

[0039] With the apparatus complete with all the systems: the plasmareactor (FIG. 1); the system of advancing and joining the graphite rods;the gas blower with the closed helium recirculation loop; and theentrapment system for the fullerene-containing soot, exhaustiveoperational tests have been carried out at the following parameters:

[0040] uninterrupted reactor operation time: 10 hours;

[0041] number of 12 mm diameter graphite rods vaporized in the reactor:7 pieces;

[0042] arc current: 320 A;

[0043] helium pressure in the reactor: 300 mm of mercury;

[0044] moving electrode advancement rate: 10 mm/min;

[0045] helium flow rate through the closed recirculation loop: 200cm²/min;

[0046] the glow discharge current: 10 mA; and

[0047] the electric separator voltage: 8 kV.

[0048] After the test:

[0049] bag filter contained 923 g of the soot;

[0050] fullerene content in the soot determined by the standard methodof toluene solution absorption was 10.5-11.0%.

1. A method of manufacturing fullerene-containing soot by vaporizinggraphite in an electric arc burning between two axially aligned graphiteelectrode in an inert gas atmosphere, removing the products formed inthe electric arc with an inert gas and their subsequent deposition inthe form of fullerene-containing soot, comprising the steps of passingthe graphite electrode being fed into the electric arc zone through aglow discharge zone and removing said products from the electric arczone by an annular stream of inert gas of a specified radius alignedcoaxially with said electrodes.
 2. A method according to claim 1comprising: simultaneously rotating said annular stream of inert gasaround the axis of said electrodes.
 3. A method according to claim 1comprising: applying to the graphite electrode being fed into saidelectric arc zone a d.c. electrical voltage and alternating its polaritybetween positive for 2-15 minutes and negative for 1-5 minutes.
 4. Amethod according to claim 1 comprising: blowing the annular stream at adistance of R>45 mm from the axis of said electrodes.
 5. An apparatusfor manufacturing fullerene-containing soot incorporating a plasmareactor in the form of hermetically sealed cylindrical chamber, an inertgas circulation system, a means for collecting the fullerene-containingsoot, two graphite rod-like electrodes, the first electrode being firmlyfixed in a cooled current feedthrough and the second electrode, mountedin another cooled current feedthrough, that can be moved in axialdirection, comprising: an additional chamber for degassing said movinggraphite electrode in a glow discharge, an inert gas recirculationsystem provided with an annular-slit nozzle aligned coaxially withelectrodes and a means for collecting the fullerene-containing sootcomprising an electric separator at the inlet of said inert gasrecirculation system.
 6. An apparatus according to claim 5 comprising ameans for joining further graphite rods to the moving graphiteelectrode.
 7. An apparatus according to claim 5 comprising a means forfeeding the movable graphite electrode to said reactor.
 8. An apparatusaccording to claim 5 wherein the internal radius of said nozzle is equalto R>45 mm.
 9. An apparatus according to claim 5 wherein the outlet ofsaid nozzle is positioned at a distance of 30-40 mm from the crosssection through the midpoint of the interelectrode gap.
 10. An apparatusaccording to claim 5 comprising a deflector mounted in the nozzle. 11.An apparatus according to claim 10 wherein said deflector is made in theform of strips mounted in the nozzle at an acute angle to its axis. 12.An apparatus according to claim 10 wherein said deflector is made in theform of guiding blades.